Ortho- and Poly-phosphate in Liquid Starter Fertilizer: An Agronomic Difference?

It seems like a funny time of year to debate the agronomic differences between ortho- and poly-phosphates in liquid starter fertilizers. I have been asked this question before, but the question has arisen again.

So what are the differences, and, from a practical perspective, does it matter?

Article Summary

Liquid starters that contain 80-to-100% phosphorus (P) in the ortho-phosphorus form are not agronomically necessary.

Some liquid starters contain 40-to-60% poly-phosphates. However, under most soil conditions, 50% or greater of the poly-phosphates will be hydrolyzed (converted) to plant-available ortho-phosphates in just a few days.

Independent research does not show greater corn yield when P is supplied in the ortho- vs. poly-phosphate form.

Temperature and Phosphate Hydrolysis

Given that ortho-phosphates are much more plant-available than poly-phosphates, an initial glance would lead you to believe starter fertilizers that contain mostly ortho-phosphates may be superior to those that do not.

Thankfully for us, water soluble, liquid poly-phosphates tend to convert to ortho-phosphates rather rapidly when applied to soils. This conversion from poly- to ortho-phosphates is an enzyme-mediated hydrolysis reaction. Therefore, as the soil temperature rises so, too, does the rate at which the conversion takes place.

Even though the hydrolysis process is slower at cool soil temperatures, Chang and Racz (1977) generally found greater than 50% of the poly-phosphates to be converted to the ortho- form in as few as 48 hours at soil temperatures as low as 41°F (Figure 1).

An example “pop up” starter for corn

So getting back to the nuts and bolts of the question, does the percentage of ortho- vs poly-phosphates in liquid starters have any real implications? Using a 6-24-6 starter fertilizer source, and corn as the crop planted in April, let’s see if not having 100% of the P in the ortho- form should even matter.

Given a very conservative 6-24-6 “pop-up” starter fertilizer application rate of 10 lbs/ac of actual P—so about 3.7 gal/ac—and a soil temperature of 50°F at planting, we would expect at least 7.5 lbs of that P to be in the ortho- form within 2 days after planting. We reach this conclusion because about 50% of the P in 6-24-6 is already in the ortho- form—so 5 of the original 10 lbs—and we would expect at least half of the remaining 5 lbs of P in the poly- form to be converted to the ortho- form within about 2 days, bringing us to 7.5 lbs of easily plant available P shortly after the seed was put in the ground.

So, is 7.5 lbs of P enough to meet the initial needs of seedling corn? Well, according to the latest research on modern corn hybrids, V5 corn accumulates less than 5 lbs of P per acre (Bender et al., 2013). By these estimations, corn P mineral nutrient requirements could be met until at least V5 on the plant available P in the modest (3.7 gal/ac) 6-24-6 starter fertilizer application.

Warming soils increase the rate of hydrolysis and P mineralization

Some may argue that portions of the P in initial starter fertilizer applications will be fixed by the soil and made unavailable to growing plants. This is true in some cases, but I would argue that banded applications tend to be pretty efficient. Moreover, by V5, the soil has warmed considerably, allowing organic forms of P to be mineralized and to become plant-available.

Along with warming soil causing organic P availability, this would further promote the hydrolysis of the remaining poly-phosphorus in the initial liquid starter application. In addition, by V5, we would expect corn to have transitioned almost entirely to the nodal root system, and thus, would be exploring soil resources other than those provided by the initial starter application.

Independent Research

In addition to it not making good sense, the research on this topic conducted by independent sources appears not to support this idea (Table 1) (Frazen and Gerwing, 1997). Some new and independent research would even suggest that ammonium poly-phosphate results in increased P uptake when compared to fertilizer sources that contain more ortho-phosphates (Table 2). The increased P uptake with ammonium poly-phosphate seems to be most prevalent in soils with the ability to fix substantial amounts of fertilizer P. In these cases, we believe keeping P temporarily in the poly form reduces the amount of fertilizer P that can be fixed into forms that are more permanently unavailable for plant uptake (Torres-Dorante et al., 2006). A clear topic of interest, but one we will save for a different time.